1. Field of the Invention
The present invention relates to a cathode for an electronic flash tube. More particularly, the present invention relates to an improvement of a cathode for an electronic flash tube capable of being manufactured with ease and with constantly high quality.
2. Description of the Prior Art
A camera incorporates an electronic flash device, which is constituted of a flash tube, a main capacitor, and a trigger capacitor. In the flash tube is enclosed xenon gas (Xe). In the inactive state of the flash tube, the resistance between an anode and a cathode of the flash tube is so high that electricity stored in the main capacitor is kept from being discharged. In synchronism with an opening movement of a shutter of the camera, the trigger capacitor discharges. A current of electricity is discharged at a high voltage, which ionizes the xenon gas so as to reduce the resistance between the anode and cathode. In response to the reduction of the resistance, the electricity stored in the main capacitor is discharged in the flash tube to cause the flash device to emit a flash of light.
The cathode to be incorporated in the flash tube must be able to emit a great number of electrons instantaneously within the flash tube. It is conventional to include cesium (Cs) in the cathode, because cesium promotes electron emission. To include cesium in the cathode, there is a conventional method in which a cesium coating is applied on the cathode. According to this known method, a cesium compound such as cesium carbonate or cesium oxalate is dissolved in water or alcohol. A base metal material, e.g. nickel (Ni), for the cathode is immersed in the cesium compound solution, in order to coat the base metal material with the cesium compound.
A problem of the cesium coating method lies in the great number of steps for manufacturing, and in complicated operation. The cesium compound solution must be prepared. The immersion of the base metal into the solution must be associated with subsequent processes, such as drying process or a surface activating process, for finishing the cathode. It is difficult to keep the quality of successively manufactured cathodes stably constant, because differences between the numerous cathodes are great regarding performance, such as differences in longevity of the flash tube incorporating each cathode, and differences in the minimum voltage for flash emission of the flash tube. It is also difficult to keep regular the proportions of compounds, including the cesium compound. If too little cesium compound is included in the cathode, the resulting flash tube emits an insufficient flash. If too much cesium compound is included, the resulting flash tube will not have a long life.